It's really just a matter of sitting around with friends and brainstorming. The only thing that really matters is if the domain name is open. Having a .com domain isn't as important as it once was. You can also use tools like http://www.panabee.com/

Just be careful when you're checking the domain name. I've heard of companies (I won't mention any because I'm not 100% sure it's true) that will purchase your domain name in a few days if you don't buy it. So if you find something open and even think you'll use it, spend the $10 now and secure it. My $0.02...